This invention relates to an audio system that effectively adds an ambience or diffuseness to the sound field generated within a given listening space, thereby increasing the apparent size or spaciousness of the listening space. This spatial effect is obtained by utilizing at least one additional speaker or other sound radiator, over and above the primary speakers employed to reproduce music or other sound effects within the listening space. The spatial effect signal employed to drive the additional speaker is developed by a transversal filter that effectively scales, delays, and recombines the original signal.
In music reproduction in the home, the performance of either a monophonic or a stereophonic audio system is frequently unsatisfactory in that the sound effects normally produced in a concert hall or like listening space are not effectively reproduced. This is particularly true for musical performances reproduced from phonograph records, tapes, radio broadcasts, or other sources. Somewhat similar problems are also encountered in outdoor concert areas and in large auditoria, where the overall effect of a local musical performance is not as pleasing as in a small concert hall or like facility with good acoustic characteristics. Multipurpose auditoria and other large halls are frequently designed for speech and require artificial sound enhancement for improvement of musical productions.
One technique that has been used to compensate for a lack of spatial effect comprises the addition of one or more auxiliary speakers, to which the primary audio signal is supplied with some delay. If the delay is substantial, however, distinct and objectionable echoes are heard by many listeners, particularly for transient sounds. Reducing the delay, on the other hand, minimizes the spatial effect, often to the point at which little or no improvement is achieved.
Another known arrangement for introducing spatial effects in an audio system comprises the use of an additional speaker, driven by an audio signal translated through a filter that employs both delay and feedback. In its simplest form, the output of a delay device is attenuated and fed back to the input, at less than unity gain, affording an output signal having an impulse response which comprises an indefinite series of evenly spaced pulses of progressively decaying amplitude. Such feedback filters are referred to in this specification as "recursive filters". When employed for enhanced spatial effect, recursive filters present substantial problems of poor frequency response, obtrusive echoes, and directional distortion.
An improvement on the simple recursive filter is described in Logan et al. U.S. Pat. No. 3,110,771, in which the recursive filter is combined with an undelayed transmission channel, utilizing specific gain relationships in both the delayed and undelayed channels of the filter. This recursive filter circuit can be constructed to have a flat frequency response, and produces enhanced spatial effects through the addition to the filter output of a specific amount of the original undelayed audio signal. However, this kind of recursive filter produces a highly peaked delay-frequency characteristic in many instances, and tends to produce a barrel-like sound due to an evenly spaced repetition of the signal with time, extending for a substantial period after termination of the primary audio signal. That is, the indefinite continuation of impulse response, using a recursive filter, even of this improved kind, affords a continuation effect that is frequently objectionable and may distort perception of the direction of origin of the sound.
Transversal filters have occasionally been utilized to generate spatial effect signals used to enhance the ambience and diffuseness of sound within a given listening space. A transversal filter has a finite impulse response, as contrasted with the indefinite impulse response to a recursive filter, so that some of the echo or continuation effects of recursive filters are not presented. Transversal filters, however, have also exhibited serious shortcomings and faults in sound quality. Thus, spatial effect audio systems employing transversal filters, as known in the art, have usually exhibited frequency responses with high peaks and dips, resulting in poor sound quality. Another fault of these systems has been the production of perceptible individual echoes, again resulting in degradation of the reproduced sound.
These problems result from inexact methods of choosing the delay values, scaling values, and proper combinations of the signals. The present invention is based in part upon the discovery that quite specific values of delay, scale, and combination are required to produce a uniform frequency response and improved spatial effects when employing a transversal filter. In attempting to overcome these problems, more complex filters with greater numbers of delay intervals have been employed, as have parallel combinations of such filters. Some degree of improvement can be obtained by these techniques, but complexity and cost are high.